The present invention relates generally to viscosity index (VI) improvers and, more particularly, to a one step method for making multifunctional VI improvers whereby the resulting product exhibits improved antioxidancy and dispersancy properties and is easy to pour point depress.
One of the known types of VI improvers consists of an ethylene-propylene-(grafted)methacrylate terpolymer. Generally, this VI improver is made in a two-step reaction sequence. More specifically, in a first step, an ethylene and propylene monomer are copolymerized to provide an ethylene-propylene copolymer. In a similar manner, ethylene-propylene-diene containing terpolymers can be provided in the first step. In any event, the polymerization reaction occurs in the presence of a Ziegler-Natta catalyst. In a second step, a methacrylate constituent is grafted onto the backbone of the copolymer or diene containing terpolymer produced in the first step. Free radical catalysts and/or heat must be employed in the second step in order to initiate the grafting reaction.
There are several shortcomings associated with the aforedescribed known method of preparing VI improvers. Specifically, in addition to providing methacrylate grafted terpolymers, a certain portion of the methacrylate monomer will homopolymerize resulting in a methacrylate polymer, which is undesirable for several reasons. First, the reaction product as well as the final oil blend are opaque, hazy and have a tendency to gel. This phenomenon is problematic to the skilled lubricant chemist and is particularly prevalent where the VI improver has a high ethylene:propylene molar ratio. Second, the reaction product includes a fraction which is insoluble or, at best, soluble in the oil with great difficulty. This disadvantage is also attributed to the crosslinking which occurs when the known method is employed. Third, the homopolymerization of the methacrylate monomer renders the known method inefficient and uneconomical, inasmuch as the methacrylate monomer is wasted, where a multifunctional VI improver is desired, when it cannot be grafted onto the ethylene-propylene backbone. The economic liability of the known method is further ascribed to the necessity to produce the VI improver in two steps and by using two distinct catalyst systems.
It is, therefore, our understanding that a one-step, economic and efficient method for making a VI improver which overcomes those shortcomings identified above is highly desired by those skilled in the art and has heretofore been unavailable.
Thus, it is an object of the present invention to provide a one-step polymerization method of preparing a multifunctional VI improver which impacts dispersancy and antioxidancy properties to lubricating oils.